In the process of drilling oil and gas wells, drilling fluid (also known as drilling mud) is injected through the drill string to flow down to the drill bit and back up to the surface in the annulus between the outside of the drill string and the wellbore to carry the drill cuttings away from the bottom of the wellbore and out of the hole. The drilling fluid is also used to prevent blowouts or kicks when the wellbore is kept substantially full of drilling fluid by maintaining head pressure on the formations being penetrated by the drill bit. A blowout or kick occurs when high pressure fluids such as oil and gas in downhole formations are released into the wellbore and rise rapidly to the surface. At the surface these fluids can potentially release considerable energy that is hazardous to people and equipment. The drilling fluids used for drilling oil and gas wells have been developed with weighting (densifying) agents to provide sufficient head pressure to prevent the initial release of high pressure fluids and gases from the formation. However, density alone does not solve the problem as the drilling fluid may drain into one or more formations downhole lowering the volume of drilling fluid in the hole and, thus, head pressure for the wellbore. The situation where drilling fluid is draining into one or more formations is called “lost circulation” or sometimes by other terms, such as “seepage loss” or simply “fluid loss” depending on the extent and rate of fluid volume losses to the formation.
Lost circulation and stuck pipe are two of the most costly problems faced while drilling oil and gas wells. To reduce the likelihood of lost circulation, particles of “lost circulation material” (commonly called “LCM”) are added to drilling fluids to plug the formations into which the drilling fluid is being lost. It is a simple and elegant solution in that the particles flow toward the leaking formation carried by the drilling fluid and then collect in the leaking formation at the side of the wellbore.
One type of lost circulation material is granular lost circulation material, which is a material chunky in shape and prepared in a range of particle sizes. Ideally, granular LCM should be insoluble and inert to the mud system in which it is used. Examples of granular LCM are ground and sized limestone or marble, wood, nut hulls, Formica laminate, corncobs and cotton hulls. Ground and sized marble can be desirable as a LCM because of its low cost and acid solubility. The latter allowing for removal of the LCM upon completion of the drilling and/or well completion operations. Unfortunately, granular LCM, in general, and marble, in particularly, is subject to degradation of particle size under shear stress such as it experiences downhole in well drilling and completion operations. Such degradation of particle size can adversely affect the granular LCM's function in the wellbore.